This invention relates to a sensor for detecting low concentrations of silane gas in the air and to a method of manufacturing the same.
In semiconductor factories, chemical factories, laboratories, and the like, large quantities are used of certain gases which can spontaneously ignite and combust upon contacting or mixing with air or other gases even when existing in concentrations of a few percent. Examples of these gases are monosilane (SiH.sub.4), dichlorosilane (SiH.sub.2 Cl.sub.2), trichlorosilane (SiHCl.sub.3), phosphine (PH.sub.3), diborane (B.sub.2 H.sub.6), and arsenic hydride (AsH.sub.3). There are increasing incidents of fires caused by the leaking of such gases into the atmosphere followed by their spontaneous ignition. For this reason, there is a great need for a gas sensor which can detect low concentrations of these gases. However, up to the present time, no such sensor has been developed.
In Japanese Patent Application No. 57-226510, the present inventors disclosed as element for sensing carbon monoxide prepared by mixing stannic oxide (SnO.sub.2), antimony oxychloride (SbOCl), and chloroplatinic acid (H.sub.2 PtCl.sub.6) so that the molar ratio of Sb to Sn is 0.02-0.08 and the molar ratio of Pt to Sn is 0.02-0.10 and calcining the mixture in air or in an oxidized antimony atmosphere at 600.degree.-850.degree. C.
In the course of experiments, the present inventors discovered that if a heating means were added to the carbon monoxide sensor described above and the sensor were thereby heated to 260.degree. C., the resulting apparatus had a high selectivity for monosilane gas (SiH.sub.4).
Specifically, in their experiments, the present inventors prepared a mixture of SnO.sub.2, H.sub.2 PtCl.sub.6, and SbOCl with a Pt/Sn molar ratio of 0.04 and an Sb/Sn molar ratio of 0.06 and coated this mixture on two alumina porcelain tubes each having a pair of electrodes. The tubes were then calcined at 700.+-.5.degree. C. for 15 minutes either in air or in a quartz tube containing an oxidized antimony gas prepared by calcining 4.0 mg of SbOCl. Electrical heating means were then inserted into the alumina porcelain tubes and a voltage was applied to the heaters to heat the tubes to 260.degree. C. The electrical resistance between the electrodes of each tube was then measured in clean air at 25.degree. C. and then again in each of eight different samples of air at 25.degree. C. each containing 100 ppm of either CO, CH.sub.4, C.sub.2 H.sub.4, C.sub.2 H.sub.6, H.sub.2, NH.sub.3, EtOH, or SiH.sub.4. Measurement in SiH.sub.4 was carried out last. The results of these measurements are shown in Table 1 in the form R.sub.o /R.sub.g, where R.sub.o is the initial electrical resistance in air and R.sub.g is the electrical resistance in a particular gas mixture. In the table, Sensor A is the sensor formed using the tube which was calcined in air and Sensor B is the sensor formed using the tube which was calcined in the aforementioned oxidized antimony gas.
TABLE 1 ______________________________________ SENSOR A SENSOR B ATMOSPHERE R.sub.o /R.sub.g R.sub.o /R.sub.g ______________________________________ CO 100 ppm 1.2 1.3 CH.sub.4 100 ppm 1.1 1.2 C.sub.2 H.sub.4 100 ppm 1.3 1.1 C.sub.2 H.sub.6 100 ppm 1.2 1.1 H.sub.2 100 ppm 1.2 1.5 NH.sub.3 100 ppm 8 7 EtOH 100 ppm 24 25 SiH.sub.4 100 ppm 145 130 ______________________________________
The initial resistance in air R.sub.o was 135 kilohms for Sensor A and 74 kilohms for Sensor B. It was found that exposure to the first seven gases in the table did not significantly effect the resistance in air, but that after exposure to SiH.sub.4 gas, the resistance in air (R.sub.air) upon the second measurement fell to 28 kilohms for Sensor A and to 16.5 kilohms for Sensor B.
After the second measurement of resistance in clean air, the resistance in each gas was again measured. This time, for Sensor A, R.sub.air /R.sub.g was 36 in SiH.sub.4, 5.8 in EtOH, and 1.1-3.6 in the other gases, while for Sensor B, R.sub.air /R.sub.g was 33 in SiH.sub.4, 6.3 in EtOH, and 1.1-3.8 in the other gases.
For the third and subsequent measurements of resistance, for both Sensor A and Sensor B the resistance in air R.sub.air showed almost no change between measurements, R.sub.air /R.sub.g in SiH.sub.4 was 18-27, and the values of R.sub.air /R.sub.g in the other gases were substantially the same as or lower than those given above.
The above experiment was repeated using different calcining temperatures and different compositions for the mixture coated on the tubes. The value of R.sub.air /R.sub.g in SiH.sub.4 was 110-300 for the first measurement, 30-50 for the second measurement, and 16-35 for subsequent measurements, while the values of R.sub.air /R.sub.g in other gases were 1/4-1/6 of the values in SiH.sub.4.
The silane gas sensor achieved by adding a heater to the carbon monoxide sensing element of Japanese Application No. 57-226510 and heating the apparatus at 200.degree.-400.degree. C. thus has a high selectivity for gaseous silanes. Although the change over time of the value of R.sub.air /R.sub.g for gaseous silanes is initially large, if the sensor is once exposed to gaseous silanes, the change over time of R.sub.air /R.sub.g becomes small and tends to reach a stable value.